Photovoltaic power plants employ solar cells to convert solar radiation to electrical energy. Photovoltaic power plants also include photovoltaic inverters (“inverters”), which convert direct current (DC) generated by the solar cells to alternating current (AC) suitable for delivery to a point of interconnect with a utility grid through a network of transformers and transmission lines. Inverters are often employed in inverter stations that comprise multiple inverters connected to a single multiple-winding medium voltage step-up transformer, which in turn is connected to a medium voltage grid.
Arc flash is a serious workplace hazard when working on inverters, such as during maintenance. Mitigating arc-flash hazard in inverter stations poses several design challenges because the utility grid to which the inverter stations are connected serve as large fault current sources, leading to high arc-flash energies within the inverter stations during arc faults. In addition, effective commissioning and maintenance activities often require full internal access to inverters while the inverters are powered ON and connected live to the utility grid. The documents IEEE 1584-2002 by the Institute of Electrical and Electronic Engineers and NFPA-70E by the National Fire Protection Association provide guidelines to analyze and estimate the arc-flash energy at various locations within the electrical systems such as AC inverter stations and to determine the appropriate personal protective equipment (PPE) required for protection against potential arc flash events.
For generic, i.e., not necessarily for photovoltaic applications, electric equipment, arc flash mitigation solutions may include reducing arc current, increasing the working distance, and reducing the clearing time. These solutions, however, may be difficult to achieve or inadequate to protect workers at inverter stations.
Certain protective devices are current limiting by design. By limiting or reducing the current available for an arc fault, the corresponding incident energy is reduced during fault-clearing times that are typically short in duration (e.g., 1-3 cycles). Fault currents at these protective devices must be in the current limiting range for them to be effective. The potential problem of this solution is that below the fault current limit, the clearing time goes up significantly and, therefore, the incident energy level may exceed workable levels for a range of grid operating conditions of a photovoltaic power plant.
Increasing the working distance will significantly reduce the incident energy level because the incident energy is proportional to the square of the distance in open air. Working distance can be increased by using remote operating devices and extension tools (e.g., hot-sticks). However, in the case of inverter stations, many maintenance or commissioning activities need to have internal access to the inverters while the inverters are powered ON and connected live to the utility grid. Therefore, increasing the working distance may not be practical in inverter stations.
One popular method of reducing clearing times is to lower the current setting of the protective device, such as a circuit breaker. A disadvantage of this solution, in particular for inverter stations, is that the level to which the current setting can be lowered is limited for normal operating conditions due to the need to coordinate the breaker tripping characteristics with equipment protection needs and avoiding normal short-term transient currents from tripping the breaker.
Other solutions for mitigating arc flash hazards in general include provision of bus differential protection and zone selective interlocking, the specific implementations of which are highly system-dependent, typically complex, and not cost effective.